Mid-wheel drive wheelchairs have front wheels, rear wheels, and drive wheels arranged between the front wheels and the rear wheels. The front and rear wheels may typically be caster-type wheels.
In comparison to front-wheel and rear-wheel driven wheelchairs, mid-wheel drive wheel chairs are sometimes considered to present a better manoeuvrability, mainly in terms of a lesser turning radius. In addition to manoeuvrability, some other aspects when designing and configuring a wheelchair are stability, ride comfort and the ability for a wheel suspension assembly to ensure that all wheels are in contact with the ground at all times. Stability may in one aspect be defined as the ability of preventing tipping of the wheelchair about its pitch axis, e.g. tipping over in the forward or rearward direction, or lifting of rear or front caster wheels when driving on a ramp or a slope.
It is especially important that the drive wheels maintain traction against the ground since the wheelchair otherwise will lose its ability to be driven, in any direction. The risk for loss of tractive force increases when operating the wheelchair on a non-planar surface, e.g. on uneven ground. The phenomena when such a situation occurs for a mid-wheel drive wheelchair is sometimes referred to as high-centring, meaning that at least one of the drive wheels has lost contact with the ground. There is also an increased risk of losing traction when entering or leaving a slope, downhill or uphill.
Comfort may in one aspect be defined as the level of vibrations and/or impact transferred from the ground to the user. A low level of transferred vibrations is experienced as more comfortable by the user.
In order to improve the ability to maintain traction to the ground, the swing arms, i.e. arms to which the wheels are mounted, of a wheelchair suspension are preferably connected to each other. For user comfort, on the other hand, it is preferable to have a fully independent suspension where one wheel movement does not affect any other.
US2008/0083573 addresses issues relating to high-centring or traction loss of the drive wheel. Hereto, US2008/0083573 discloses a vehicle including a chassis, a driving unit including a mounting seat pivoted to the chassis, a driving member mounted on the mounting seat, and two driving wheels mounted rotatably on the mounting seat and driven by the driving member. The vehicle further includes two front wheel units, each including a front lever having first and second ends and pivoted to the chassis, a front wheeled member connected to the first end, and a first cushioning member pivoted to the second end and the mounting seat for urging the front lever to rotate, and two rear wheel units, each including a rear lever having first and second ends and pivoted to the chassis, a rear wheeled member connected to the first end, a second cushioning member pivoted to the first end and the mounting seat, and a linkage pivoted to the second end of the rear lever and the mounting seat.
US2014/0339004 aims at providing a suspension for a wheelchair which facilitates for a wheelchair to traverse obstacles by aiding the front casters in overcoming the obstacle. Hereto, this document discloses a suspension for a wheelchair which includes a drive assembly and a front caster pivot arm. The drive assembly and the front caster pivot arm are pivotally connected to the frame such that the drive assembly and the front caster pivot arm are pivotable relative to each other. The drive assembly and the front caster pivot arm are coupled to enhance the vehicle's ability to traverse obstacles.